In many broadband applications such as Asymmetric Digital Subscriber Line (ADSL), a receive signal is received in a line driver/receiver circuit at the same time as a transmit signal is sent over one and the same transmission line.
The transmit signal that is much stronger than the receive signal that is damped by the transmission line, will be coupled with its distortion into the receiver and degrade the signal-to-noise ratio (SNR) of the receive signal.
Since the bitrate of the receive communication is decided in advance during a so-called training sequence by the detected SNR for the receive signal, also the bitrate of the receive signal will degrade.
To solve this problem, it is known to use echo cancellers.
FIG. 1 on the appended drawing is a schematic illustration of an embodiment of a known line driver/receiver circuit with an echo cancellation bridge.
The line driver 1 is connected with its output terminals via equal drive impedances ZT to a load impedance ZL that comprises a transformer connected to a transmission line that is connected to a subscriber station with a line driver/receiver circuit for transmitting and receiving signals to and from the transmission line.
The receiver 2 is connected with its input terminals to the load ZL for simultaneously receiving a receive signal from the transmission line.
To cancel the transmit signal and distortion on the input terminals of the receiver 2, the input terminals of the receiver 2 are connected to the load via equal resistors R1 and to respective output terminal of the line driver 1 via equal resistors R2.
In the embodiment in FIG. 1, the sum of the drive impedances 2ZT shall have the same impedance value as the load impedance ZL.
Since the output impedance of the line driver 1 is very low compared to the drive impedances ZT, no receive signal at all will be present at the interface between the line driver 1 and the resistor bridge R1, R2. However, at the interface between the resistor bridge R1, R2 and the load ZL both transmit and receive signals will be present.
It can be shown that by applying the resistor bridge R1, R2, the transmit signal can be totally canceled at the input terminals of the receiver 2 while the receive signal is still present.
In line drivers with active termination impedance, the drive impedance ZT in FIG. 1 would be set by a feedback loop inside the line driver 1. The advantage of such line drivers is that the output signal levels can be lowered for a given transmit power over the load ZL. Hereby, it will be possible to reduce the supply voltage and, consequently, the power consumption.
However, also in such line drivers, there is a need of good echo cancellation of transmit signals at the receiver input.